To measure the rheology of various liquids, rheometers were designed in many different types, of which a typical rheometer can measure both the rheology of cement-based materials (e.g. concrete), and also the viscosity (η) and yield stress (τy), as illustrated in two-point test in FIG. 1. The rheometer 100 includes a drum 110 to accommodate concrete, and a rotation shaft 120 driven by a motor 130 and connected to a plurality of vanes 121. This theory was initiated by G H Tattersall, and the rheology of concrete is simulated as Bingham fluid, for which a shear stress similar to static friction would exist prior to the initiation of shear strain. Only when the shear stress reaches a specific value, the shear strain rate ({dot over (γ)}) starts to change. The linear relationship between shear stress (τ) and shear strain rate is shown in Eq. (1). By using two-point test apparatus, the relationship between measured torque (T) and rotational speed (N) can be converted into shear stress and shear strain rate shown in FIG. 2. The relationship is shown in Eq. (2).τ=τy+η{dot over (γ)}  (1)T=g+hN  (2)Two indicators, the slope (h) and the intercept (g) of the T-N curve are used to determine the workability of concrete. The relationship among g, h and G, K are shown in Eqs. (3) and (4).
                              τ          y                =                              K            G                    ⁢          g                                    (        3        )                                η        =                              1            G                    ⨯          h                                    (        4        )            
whereby G and K are the rheological constants of the vane. However, the vane 121 of rheometer 100 cannot be replaceably adjusted. And, the theory of rheological constant K shows that, an optimal spacing (minimum error) is achieved when the ratio (r1/r0) of radius (r1) of vane to radius (r0) of drum is more than 0.97. But, this spacing is suitable only for fluids not containing solid particles of certain sizes, such as coarse aggregates in concrete. When applied to concrete, vane 121 may be jammed if theoretical value of ratio (r1/r0) is set as 0.97. Thus, the ratio (r1/r0) of the radius of vane 121 to the radius of drum 110 is between 0.6˜0.85, to avoid jams. Besides, a negative yield stress for self-consolidating concrete (SCC) has been found by using this technique. This is because SCC has a lower yield stress and the calculation of the regression line affects the intercept of the torque axis in such a way that it sometimes gives a negative value. The negative values are attributed to the error in the extrapolation process and have no real physical meaning. Further research is needed to develop a new technique to determine the yield stress of SCC.